Fault Detection Circuits and Methods for Drivers

ABSTRACT

A fault detection circuit includes a short circuit comparison circuit which has a first input connected to the source of the second NFET, a second input, and an output. The circuit includes an over-current comparison circuit which has a first input connected to the source of the second NFET, a second input, and an output. The circuit includes a voltage divider circuit which has a first terminal connected to first input of the short circuit comparison circuit, a second terminal connected to the first input of the over-current comparison circuit, and a third terminal connected to a ground terminal. The circuit includes a delay circuit which has an input connected to the output of the over-current comparison circuit and has an output.

TECHNICAL FIELD

This description relates generally fault detection circuits and methods.

BACKGROUND

In switch mode regulators (e.g., buck regulator, boost regulator), a switch such as an n-channel field effect transistor (NFET) or a p-channel field effect transistor (PFET) is turned on and off at a high frequency to provide an output voltage. By varying the ratio of “on” to “off” time of the switch, the output voltage is regulated. The ratio of the “on” to “off” time is known as a duty cycle.

If the current drawn by a load coupled to a switch mode regulator rises suddenly, the current through a switch may rise above an over-current threshold, thereby causing an over-current fault. In the event of an over-current fault, the switch is turned off for the remainder of the on time period of the current cycle. However, because the over-current fault may only be transitory (e.g., few cycles) and the fault may clear itself when the current drawn by the load returns to a normal level, the switch is preferably turned on at the beginning of the next cycle and normal switching operation is resumed. Thus, if an over-current fault is detected, the switch is operated on a “cycle-by-cycle” mode.

If there is a short circuit in the regulator due to a control failure or an electrical failure, the current may shoot through the over-current threshold and also above a short circuit threshold, causing a short circuit fault. For example, an NFET may be rated with an over-current threshold of 70 A and a short circuit threshold of 100 A. Because a short circuit fault is not typically transitory and the switch can only tolerate a short circuit fault for a very brief period (e.g., 250 ns), operating the switch on a “cycle-by-cycle” mode can damage the switch. If a short circuit fault occurs, the switch is preferably completely latched-off and held in a latched-off state.

Existing switch mode regulators generally are not designed for both the “cycle-by-cycle” mode and the “latch-off” mode. Irrespective of the type of fault, existing regulators are designed to be run on a “cycle-by-cycle” mode or they can be completely latched-off. If a regulator provides the “latch-off” feature as its fault protection, completely latching-off the switch in the event of an over-current fault is not optimal. Conversely, if a regulator provides the “cycle-by-cycle” feature as its fault protection, running the switch on the “cycle-by-cycle” mode in the event of a short circuit fault will likely result in a catastrophic outcome.

SUMMARY

In one aspect, a fault detection circuit includes a first NFET which has a drain, a source, and a gate. The circuit includes a second NFET which has a drain connected to the drain of the first NFET, a gate connected to the gate of the first NFET, and a source. The circuit includes a sense resistor which has a first terminal connected to the source of the second NFET and a second terminal connected to the source of the first NFET. The circuit includes a current source configured to provide a reference current at an output. The circuit includes a short circuit comparison circuit which has a first input connected to the source of the second NFET, and includes a second input and an output. The circuit includes an over-current comparison circuit which has a first input connected to the source of the second NFET, and includes a second input and an output. The circuit includes a voltage divider circuit which has a first terminal connected to first input of the short circuit comparison circuit, a second terminal connected to the first input of the over-current comparison circuit, and a third terminal connected to a ground terminal. The circuit includes a delay circuit which has an input connected to the output of the over-current comparison circuit and has an output.

In an additional aspect, a sensed voltage proportional to a current through the first NFET is provided across the sense resistor. The voltage divider circuit is configured to apply a short circuit limit voltage to the first terminal of the short circuit comparison circuit and to apply an over-current limit voltage to the first terminal of the over-current comparison circuit.

In an additional aspect, the short circuit comparison circuit is configured to provide a short circuit fault signal when the sensed voltage is greater than the short circuit limit voltage, and the over-current comparison circuit is configured to provide an over-current fault signal when the sensed voltage is greater than the over-current limit voltage. The delay circuit is configured to provide a delayed over-current fault signal responsive to the over-current fault signal.

In an additional aspect, the voltage divider circuit includes a variable resistor which has a first terminal connected to the first input of the short circuit comparison circuit and a second terminal connected to the first input of the over-current comparison circuit. The voltage divider circuit includes a resistor which has a first terminal connected to the second terminal of the variable resistor and a second terminal connected to a ground terminal.

In an additional aspect, a fault detection circuit includes a first transistor which has first and second terminals and a gate. The circuit includes a second transistor which has a first terminal connected to the first terminal of the first transistor, and has a second terminal and a gate. The circuit includes a sense resistor which has a first terminal connected to the second terminal of the second transistor and a second terminal connected to the second terminal of the first transistor, and includes a current source configured to provide a reference current at an output. The circuit includes a short circuit comparison which has a first input connected to the second terminal of the second transistor, and has a second input and an output. The circuit includes an over-current comparison circuit which has a first input connected to the second terminal of the second transistor, and has a second input and an output. The circuit includes a voltage divider circuit which has a first terminal connected to first input of the short circuit comparison circuit, a second terminal connected to the first input of the over-current comparison circuit, and a third terminal connected to a ground terminal. The circuit includes a delay circuit having an input connected to the output of the over-current comparison circuit and an output. In an additional aspect, the first and second transistors are first and second PFETs, respectively. The first and second PFETs have respective sources connected together, the first and second PFETs have respective drains connected together, the first PFET has a drain, and the second PFET has a drain connected to the first terminal of the sense resistor.

In an additional aspect, a driver circuit includes a power NFET which has a drain, a source and a gate. The driver circuit includes a first NFET which has a drain connected to the source of the power NFET, a source, and a gate. The driver circuit includes a second NFET which has a drain connected to the source of the power NFET, a gate connected to the gate of the first NFET, and a source. The driver circuit includes a sense resistor which has a first terminal connected to the source of the second NFET and a second terminal connected to the source of the first NFET. The driver circuit includes a current source configured to provide a reference current at an output. The driver circuit includes a short circuit comparison circuit which has a first input connected to the source of the second NFET, a second input, and an output. The driver circuit includes an over-current comparison circuit which has a first input connected to the source of the second NFET, a second input, and an output. The driver circuit includes a voltage divider circuit which has a first terminal connected to first input of the short circuit comparison circuit, a second terminal connected to the first input of the over-current comparison circuit, and a third terminal connected to a ground terminal. The driver circuit includes a delay circuit which has an input connected to the output of the over-current comparison circuit and includes an output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a switch mode regulator of an example embodiment.

FIG. 2 is a schematic diagram of a regulator of an example embodiment.

FIGS. 3-4 illustrate waveforms of the regulator of FIG. 2.

FIG. 5 is a flow diagram of a method of an example embodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a switch mode regulator 100 of an example embodiment.

The switch mode regulator 100 shown in FIG. 1 is a buck regulator. In other embodiments, the regulator may be a boost regulator, a buck-boost regulator, an LLC converter, or any other type of switch mode regulators.

The regulator 100 includes a high-side switch M_(H) which has a first terminal 104 connected to an input voltage terminal 106, a second terminal 108, and a gate 110. The input voltage terminal 106 can be connected to an input voltage supply V_(in).

The regulator 100 includes a high-side current sensor C_Sense_(H) which has a first terminal 116 connected to the second terminal 108 of M_(H), a second terminal 118 connected to a switching terminal SW (also known as a switching node), and an output 120. The regulator 100 includes a high-side fault detection circuit Fault_Det_(H) which has an input 124 connected to the output 120 of C_Sense_(H), and has a first output 126 and a second output 128. The regulator 100 includes a high-side driver Driver_(H) which has a first input 130 connected to the first output 126 of the high-side fault detection circuit Fault_Det_(H) and has a second input 132 connected to the second output 128 of the high-side fault detection circuit Fault_Det_(H). The high-side driver Driver_(H) has an output 134 connected to the gate 110 of M_(H).

The regulator 100 includes a low-side switch M_(L) which has a first terminal 140 connected to the switching terminal SW, and has a second terminal 142 and a gate 144. The regulator 100 includes a low-side current sensor C_Sense_(L) which has a first terminal 146 connected to the second terminal 142 of the low side switch M_(L), a second terminal 148 connected to a ground terminal 150, and an output 152. The ground terminal 150 can be connected to a ground voltage level.

The regulator 100 includes a low-side fault detection circuit Fault_Det_(L) which has an input 158 connected to the output 152 of C_Sense_(L), and has a first output 160 and a second output 162. The regulator 100 includes a low-side driver Driver_(L) which has a first input 166 connected to the first output 160 of the low-side fault detection circuit Fault_Det_(L) and has a second input 168 connected to the second output 162 of the low-side fault detection circuit Fault_Det_(L). The low-side driver Driver_(L) has an output 170 connected to the gate 144 of the low-side switch M_(L).

The regulator 100 includes an inductor L connected between the switching terminal SW and a regulator output \T_(out). A capacitor C is connected between \T_(out) and the ground terminal 150, and a load represented by R_(L) is connected between \T_(out) and the ground terminal 150.

In an example embodiment, the high-side switch M_(H) is an n-channel field effect transistor (NFET). The first terminal 104 of the NFET M_(H) is the drain 104 and the second terminal 108 of the NFET M_(H) is the source 108. The low-side switch M_(L) is also an NFET of which the first terminal 140 is the drain 140 and the second terminal 142 is the source 142.

In operation, the high-side switch M_(H) and the low-side switch M_(L) are turned on/off in a complementary way. When M_(H) is on and M_(L) is off, V_(in) is coupled to the switching terminal SW. As a result, current in the inductor L rises and flows to the load R_(L), and the capacitor C is charged. When M_(H) is off and M_(L) is on, the switching terminal SW is coupled to the ground terminal 150. As a result, the current through the inductor Li falls, but continues to flow to the load R_(L). During that time, the capacitor C supplies current to the load R_(L) to compensate for the current demanded by R_(L). Thus, when M_(H) is on, the capacitor C is charged and when M_(L) is on, the capacitor C provides the current to compensate for the imbalance due to: (a) the fall in the current through the inductor L; and (b) the current required to drive the load R_(L).

The high-side current sensor C_Sense_(H) is configured to provide a sensed voltage V_(SENSE) which is proportional to, and representative of, the current flowing through the high-side switch M_(H). The high-side fault detection circuit Fault_Det_(H) receives V_(SENSE). If V_(SENSE) is greater than an over-current limit voltage V_(SC_LIM), the high-side fault detection circuit Fault_Det_(H) generates an over-current fault signal OC_(FAULT). The high-side fault detection circuit Fault_Det_(H) applies a predetermined delay to OC_(FAULT) and provides a delayed over-current fault signal DEL_OC_(FAULT). If V_(SENSE) is greater than a short circuit limit voltage V_(OC_LIM), the high-side fault detection circuit Fault_Det_(H) provides a short circuit fault signal SC_(FAULT).

The high-side driver Driver_(H) controls the operation of the high-side switch M_(H) by applying a pulse width modulated (PWM) signal to the gate 110 of M_(H). The PWM signal has a duty cycle which describes the proportion of “on” time to the “off” time. By varying the duty cycle of the PWM signal, Driver_(H) controls the “on” time and the “off” time of M_(H).

In an example embodiment, the high-side driver Driver_(H) receives the delayed over-current fault signal DEL_OC_(FAULT) and the short circuit fault signal SC_(FAULT). If the delayed over-current fault signal DEL_OC_(FAULT) appears prior to the short circuit fault signal SC_(FAULT), the condition is considered an over-current fault, and in response Driver_(H) turns off the high-side switch M_(H) for the remaining on time period of the current cycle, but turns on M_(H) at start of the next cycle. For each occurrence of an over-current fault, Driver_(H) turns off the high-side switch M_(H) for the rest of the on time period of the current cycle, but turns on M_(H) at the start of the next cycle. Thus, M_(H) is operated on a cycle-by-cycle mode.

If the short circuit fault signal SC_(FAULT) appears before the delayed over-current fault signal DEL_OC_(FAULT), the condition is considered a short circuit fault, and in response Driver_(H) turns off the high-side switch M_(H). In an example embodiment, the short circuit fault signal SC_(FAULT) is provided to a main controller (not shown in FIG. 1). The main controller may completely shut down the regulator 100 by latching off both M_(H) and M_(L).

The low-side current sensor C_Sense_(L) is configured to provide a sensed voltage V_(SENSE) which is proportional to, and is representative of, the current flowing through the low-side switch M_(H). The low-side fault detection circuit Fault_Det_(L) receives V_(SENSE). If V_(SENSE) is greater than the over-current limit voltage V_(SC_LIM), Fault_Det_(L) generates the over-current fault signal OC_(FAULT). The low-side fault detection circuit Fault_Det_(L) applies the predetermined delay to OC_(FAULT) and provides the delayed over-current fault signal DEL_OC_(FAULT). If V_(SENSE) is greater than a short circuit limit voltage V_(OC_LIM), the low-side fault detection circuit Fault_Det_(L) provides a short circuit fault SC_(FAULT) signal.

The low-side driver Driver_(L) receives the delayed over-current fault signal DEL_OC_(FAULT) and the short circuit fault signal SC_(FAULT). If the delayed over-current fault signal DEL_OC_(FAULT) appears prior to the short circuit fault signal SC_(FAULT), the condition is considered an over-current fault, and in response Driver_(L) turns off the low-side switch M_(L) for the rest of the on time period of the current cycle, but turns on M_(L) at start of the next cycle. For each occurrence of an over-current fault, Driver_(L) turns off the low-side switch M_(L) for the rest of the on time period of the current cycle, but turns on M_(L) at start of the next duty cycle. Thus, M_(H) is operated on a cycle-by-cycle mode. If the short circuit fault signal SC_(FAULT) appears before the delayed over-current fault signal DEL_OC_(FAULT), the condition is considered a short circuit fault, and in response Driver_(L) turns off the low-side switch M_(L). In response to SC_(FAULT), the main controller (not shown in FIG. 1) may shut down the regulator 100 by latching off both M_(H) and M_(L).

FIG. 2 is a schematic diagram of: (1) the high-side switch M_(H) (or the low-side switch ML); (2) the high-side current sense circuit C_Sense_(H) (or the low-side current sense circuit C_Sense_(L)); and (3) the high-side fault detection circuit Fault_Det_(H) (or the low-side fault detection circuit Fault-Det_(L)) of an example embodiment. The high-side switch M_(H) can be a Gallium Nitride (GaN)-type NFET or a silicon-type NFET. The drain 104 of the NFET M_(H) is connected to the input voltage terminal 106. The NFET M_(H) has the source 108 and the gate 110. The high-side current sense circuit C_Sense_(H) includes NFETs M1 and M2 and a sense resistor R_(S). The NFET M1 has a drain 204 connected to the source 108 of M_(H), a source 206 connected to the switching terminal SW, and has a gate 208. The NFET M2 has a drain 210 connected to the drain 204 of M1 and has a gate 216 connected to the gate 208 of M1. The NFET M2 has a source 214. The sense resistor R_(S) has a first terminal 218 connected to the source 214 of M2 and has a second terminal 220 connected to the source 206 of M1.

In an example embodiment, the NFETs M1 and M2 are sized such that M2 conducts only a small proportion of the current through M1. By way of example, M1 may be sized 1000 times larger than M2, thus allowing M2 to conduct only 1/1000th of the current through M1. In an example embodiment, M1 and M1 are turned on and are held in the on state during operation of the regulator 100.

In operation, when M_(H) is on, an input current I_(IN) flows from V_(in) through M_(H). Since M1 and M2 are both on, the input current I_(IN) is shared between M1 and M2 based on the proportion of their sizes. If M1 is sized 1000 times bigger than M2, M2 will conduct only 1/1000^(th) of the current through M1. The portion of the current I_(IN) which flows through M2 also flows through the sense resistor R_(S), generating a sensed voltage V_(s) across R_(S). The sensed voltage V_(S) is proportional to, and representative of, the input current I_(IN) which flows through the high-side NFET M_(H).

The fault detection circuit Fault_Det_(H) includes a reference current source 230 configured to provide a reference current I_(REF) at an output 232. The fault detection circuit Fault_Det_(H) includes a short circuit comparison circuit 236 which has a first input 238 connected to the source 214 of the NFET M2 and has a second input 240 and an output 242.

The fault detection circuit Fault_Det_(H) includes an over-current comparison circuit 250 which has a first input 252 connected to the source 214 of the NFET M2. The over-current comparison circuit 250 has a second input 254 and an output 256.

The fault detection circuit Fault_Det_(H) includes a voltage divider circuit 260 which has a first terminal 262 connected to the output 232 of the reference current source 230 and connected to the second input 240 of the short circuit comparison circuit 236. The voltage divider circuit 260 has a second terminal 264 connected to the second input 254 of the over-current comparison circuit 250, and a third terminal 266 connected to the ground terminal 150.

The fault detection circuit Fault_Det_(H) includes a delay circuit 270 which has an input 272 connected to the output 256 of the over-current comparison circuit 250 and has an output 274.

The voltage divider circuit 260 is configured to apply a short circuit limit voltage VSC_(LIM) to the second terminal 240 of the short circuit comparison circuit 236 and to apply an over-current limit voltage VOC_(LIM) to the second input 254 of the over-current comparison circuit 250.

In an example embodiment, the voltage divider circuit 260 includes a variable resistor R_(VAR) which has a first terminal connected to the second input 240 of the short circuit comparison circuit 236 and a second terminal connected to the second input 254 of the over-current comparison circuit 250. The voltage divider circuit 260 includes a resistor R₁ which has a first terminal connected to the second terminal of the variable resistor R_(VAR) and a second terminal connected to the ground terminal 150.

The values of the variable resistor R_(VAR) and the resistor R₁ are selected so that the short circuit limit voltage VSC_(LIM) is representative of the short circuit current threshold of M_(H) and the over-current limit voltage VOC_(LIM) are representative of the over-current threshold of M_(H). If, for example, the short circuit current threshold of M_(H) is 100 A and the over-current threshold of M_(H) is 70 A, the resistors R_(VAR) and R₁ can be selected so that VSC_(LIM) is 100 mV and VOC_(LIM) is 70 mV.

If the sensed voltage V_(S) is greater than VO_(CLIM), the over-current comparison circuit 250 is configured to provide an over-current fault signal OC_(FAULT). If the sensed voltage V_(S) is greater than VSC_(LIM), the short circuit comparison circuit 236 is configured to provide a short circuit fault signal SC_(FAULT). In an example embodiment, the short circuit comparison circuit 236 and the over-current comparison circuit 250 are implemented with analog comparators with matched propagation delays.

The delay circuit 270 applies a predetermined time delay to the over-current fault signal OC_(FAULT) and provides a delayed over-current fault signal DEL_OC_(FAULT).

The high-side driver Driver_(H) has a first input 276 configured to receive the delayed over-current fault signal DEL_OC_(FAULT) and has a second input 278 configured to receive the short circuit fault signal SC_(FAULT). If the delayed over-current fault signal DEL_OC_(FAULT) appears prior to the short circuit fault signal SC_(FAULT), the condition is considered an over-current fault, and in response the high-side driver Driver_(H) turns off the high-side switch M_(H) for the remainder of the on time period of the current cycle, but turns on M_(H) at start of the next cycle. For each over-current fault, Driver_(H) turns off the high-side switch M_(H) for the rest of the on time period of the current cycle, but turns on M_(H) at start of the next cycle. Thus, M_(H) is operated in a cycle-by-cycle mode. If the short circuit fault signal SC_(FAULT) appears before the delayed over-current fault signal DEL_OC_(FAULT), the condition is considered a short circuit fault, and in response Driver_(H) turns off the high-side switch M_(H). The short circuit fault signal SC_(FAULT) is provided to a main controller (not shown in FIG. 2). The main controller may shut down the regulator 100 by turning off both M_(H) and M_(L).

In another example embodiment, the high-side current sense circuit C_Sense_(H) (or the low-side current sense circuit C_Sense_(L)) can be implemented with p-channel field effect transistors (PFETs). In that embodiment, M1 is a PFET whose source is connected to the source 108 of M_(H) and whose drain is connected to the switching terminal SW. Also, M2 is a PFET whose source is connected to the first terminal 218 of the sense resistor R_(S) and whose drain is connected to the drain of M1. The gates of M1 and M2 are interconnected.

FIG. 3 illustrates various waveforms when DEL_OC_(FAULT) appears before SC_(FAULT). The waveforms are: current 304 through M_(H); current 306 through the inductor L; sensed voltage V_(S) 308 across R_(S); over-current fault signal OC_(FAULT) 312 provided by the over-current comparison circuit 250; short circuit fault signal SC_(FAULT) 316 provided by the short circuit comparison circuit 236; and delayed over-current fault signal DEL_OC_(FAULT) 320 provided by the delay circuit 270.

At time T0, the high-side NFET M_(H) is turned on, thus coupling the inductor L to V_(IN). The current 304 through M_(H) rises to a level that is equal to the current flowing through the inductor L. As the current 306 through the inductor L continues to rise, V_(s) 308 rises proportionally. At time T1, V_(S) 308 rises above VOC_(LIM), thus indicating that the current 304 through M_(H) has risen above the over-current threshold limit of M_(H). As a result, the over-current comparison circuit 250 provides OC_(FAULT) 312. At time T2, the delay circuit 270 applies a predetermined delay to OC_(FAULT) 312 and provides DEL_OC_(FAULT) 320. The current 304 continues to rise through M_(H) and at time T3, V_(S) rises above VSC_(LIM), thus indicating that the current 304 through M_(H) has risen above the short circuit current threshold of M_(H). As a result, the short circuit comparison circuit 236 provides SC_(FAULT) 316. Since, DEL_OC_(FAULT) 320 appears before SC_(FAULT) 316, the condition is considered an over-current fault, and accordingly the driver DRIVER_(H) turns off M_(H) for the rest of the on time of the current cycle and operates M_(H) in a cycle-by-cycle mode.

FIG. 4 illustrates various waveforms when SC_(FAULT) appears before DEL_OC_(FAULT). The waveforms are: current 404 through M_(H); current 406 through the inductor; sensed voltage V_(S) 408 across R_(S); over-current fault signal OC_(FAULT) 412 provided by the over-current comparison circuit 250; short circuit fault signal SC_(FAULT) 416 provided by the short circuit comparison circuit 236; and delayed over-current fault signal DEL_OC_(FAULT) 420 provided by the delay circuit 270.

At time T0, the high-side NFET M_(H) is turned on. As. A result the current 404 through M_(H) rises to a level that is equal to the current flowing through the inductor L. As the current 406 through the inductor L continues to rise, V_(s) rises proportionally.

At time T1, V_(S) 408 rises above VOC_(LIM), thus indicating that the current 404 through M_(H) has risen above the over-current threshold limit of M_(H). As a result, the over-current comparison circuit 250 provides OC_(FAULT) 412. The delay circuit 270 applies a predetermined delay to OC_(FAULT) 412 and provides DEL_OC_(FAULT) 420 at time T3. However, prior to T3, at time T2, the current 404 rises above the short circuit current threshold of M_(H), causing V_(S) to rise above VSC_(LIM). As a result, the short circuit comparison circuit 236 provides SC_(FAULT) 420 at T2. Since, SC_(FAULT) 416 appears prior to DEL_OC_(FAULT) 420, the condition is considered a short circuit fault. Accordingly the driver DRIVER_(H) turns off M_(H). Also, as discussed before, a main controller may in response to the short circuit fault shut down the regulator 100.

As discussed before, the values of the resistor R₁ and the variable resistor R_(VAR) of the voltage divider circuit are selected to set the over-current limit voltage VOC_(LIM) and the short circuit limit voltage VSC_(LIM). By varying R_(VAR), the separation between VOC_(LIM) and VSC_(LIM) is varied. If the value of R_(VAR) is increased, VSC_(LIM) is moved away (higher) from VOC_(LIM), which provides less protection from catastrophic short circuit failure. Conversely, if the value of R_(VAR) is decreased, VSC_(LIM) is moved closer to VOC_(LIM), which provides increased protection from catastrophic short circuit failure but increases the frequency of regulator shut downs.

Because the sensed voltage V_(s) is proportional to the current through M_(H) (or M_(L)), the rate of change dV_(s)/dt is proportional to di/dt. If di/dt is high, dV_(s)/dt is also high. As a consequence, V_(S) rises quickly above VOC_(LIM), thereby triggering OC_(FAULT) and then quickly rises above SC_(LIM), thereby triggering SC_(FAULT) before DEL_OC_(FAULT) appears. Conversely, if di/dt is low, V_(S) slowly rises above OC_(LIM), thereby triggering OC_(FAULT), but due to the low di/dt DEL_OC_(FAULT) appears before SC_(FAULT).

FIG. 5 is a flow diagram of a method of an example embodiment. In a block 504, a sensed voltage which is proportional to a current through a transistor is provided. In a block 508, a short circuit limit voltage and an over-current limit voltage are provided. In a block 512, the sensed voltage is compared to the short circuit limit voltage and the over-current limit voltage.

If the sensed voltage is greater than the over-current limit voltage, in a block 516 an over-current fault signal is provided. In a block 520, a delayed over-current fault signal is provided by applying a delay to the over-current fault signal. If the sensed voltage is greater than the short circuit limit voltage, in a block 524 a short circuit fault signal is provided. If the short circuit fault signal appears prior to the delayed over-current fault signal, the condition is considered a short circuit fault and accordingly the transistor is latched off. If the delayed over-current fault signal appears prior to the short circuit fault signal, the condition is considered an over-current fault, and the transistor is turned off during the remainder of the current cycle, but is turned on at the beginning of the next cycle.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims. 

What is claimed is:
 1. A fault detection circuit, comprising: a first NFET having a drain, a source, and a gate; a second NFET having a drain connected to the drain of the first NFET, a gate connected to the gate of the first NFET, and a source; a sense resistor having a first terminal connected to the source of the second NFET and a second terminal connected to the source of the first NFET; a current source configured to provide a reference current at an output; a short circuit comparison circuit having a first input connected to the source of the second NFET, a second input, and an output; an over-current comparison circuit having a first input connected to the source of the second NFET, a second input, and an output; a voltage divider circuit having a first terminal connected to first input of the short circuit comparison circuit, a second terminal connected to the first input of the over-current comparison circuit, and a third terminal connected to a ground terminal; and a delay circuit having an input connected to the output of the over-current comparison circuit and an output.
 2. The fault detection circuit of claim 1, wherein a sensed voltage proportional to a current through the first NFET is provided across the sense resistor.
 3. The fault detection circuit of claim 1, wherein the voltage divider circuit is configured to apply a short circuit limit voltage to the first terminal of the short circuit comparison circuit.
 4. The fault detection circuit of claim 1, wherein the voltage divider circuit is configured to apply an over-current limit voltage to the first terminal of the over-current comparison circuit.
 5. The fault detection circuit of claim 1, wherein the short circuit comparison circuit is configured to provide a short circuit fault signal when the sensed voltage is greater than the short circuit limit voltage.
 6. The fault detection circuit of claim 1, wherein the over-current comparison circuit is configured to provide an over-current fault signal when the sensed voltage is greater than the over-current limit voltage.
 7. The fault detection circuit of claim 1, wherein the delay circuit is configured to provide a delayed over-current fault signal responsive to the over-current fault signal.
 8. The fault detection circuit of claim 1, wherein the voltage divider circuit comprises: a variable resistor having a first terminal connected to the first input of the short circuit comparison circuit and a second terminal connected to the first input of the over-current comparison circuit; a resistor having a first terminal connected to the second terminal of the variable resistor and a second terminal connected to a ground terminal.
 9. The fault detection circuit of claim 1, wherein the source of the first NFET is connected to a switching terminal.
 10. The fault detection circuit of claim 1, wherein the source of the first NFET is connected to a ground terminal.
 11. A fault detection circuit, comprising: a first transistor having first and second terminals and a gate; a second transistor having a first terminal connected to the first terminal of the first transistor, a second terminal and a gate; a sense resistor having a first terminal connected to the second terminal of the second transistor and a second terminal connected to the second terminal of the first transistor; a current source configured to provide a reference current at an output; a short circuit comparison circuit having a first input connected to the second terminal of the second transistor, a second input, and an output; an over-current comparison circuit having a first input connected to the second terminal of the second transistor, a second input, and an output; a voltage divider circuit having a first terminal connected to first input of the short circuit comparison circuit, a second terminal connected to the first input of the over-current comparison circuit, and a third terminal connected to a ground terminal; and a delay circuit having an input connected to the output of the over-current comparison circuit and an output.
 12. The fault detection circuit of claim 11, wherein the first and second transistors are first and second PFETs, respectively, the first and second PFETs have respective sources connected together, the first and second PFETs have respective drains connected together, the first PFET has a drain, the second PFET has a drain connected to the first terminal of the sense resistor.
 13. The fault detection circuit of claim 11, wherein a sensed voltage proportional to a current through the first transistor is provided across the sense resistor.
 14. The fault detection circuit of claim 11, wherein the voltage divider circuit is configured to apply a short circuit limit voltage to the first terminal of the short circuit comparison circuit.
 15. The fault detection circuit of claim 11, wherein the voltage divider circuit is configured to apply an over-current limit voltage to the first terminal of the over-current comparison circuit.
 16. The fault detection circuit of claim 11, wherein the short circuit comparison circuit is configured to provide a short circuit fault signal when the sensed voltage is greater than the short circuit limit voltage.
 17. The fault detection circuit of claim 11, wherein the over-current comparison circuit is configured to provide an over-current fault signal when the sensed voltage is greater than the over-current limit voltage.
 18. The fault detection circuit of claim 11, wherein the delay circuit is configured to provide a delayed over-current fault signal responsive to the over-current fault signal.
 19. The fault detection circuit of claim 11, wherein the voltage divider circuit comprises: a variable resistor having a first terminal connected to the first input of the short circuit comparison circuit and a second terminal connected to the first input of the over-current comparison circuit; and a resistor having a first terminal connected to the second terminal of the variable resistor and a second terminal connected to a ground terminal.
 20. The fault detection circuit of claim 11, wherein the second terminal of the the first transistor is connected to a switching terminal.
 21. The fault detection circuit of claim 11, wherein the second terminal of the first transistor is connected to a ground terminal.
 22. A driver circuit, comprising: a power NFET having a drain, a source, and a gate; a first NFET having a drain connected to the source of the power NFET, a source, and a gate; a second NFET having a drain connected to the source of the power NFET, a gate connected to the gate of the first NFET, and a source; a sense resistor having a first terminal connected to the source of the second NFET and a second terminal connected to the source of the first NFET; a current source configured to provide a reference current at an output; a short circuit comparison circuit having a first input connected to the source of the second NFET, a second input, and an output; an over-current comparison circuit having a first input connected to the source of the second NFET, a second input, and an output; a voltage divider circuit having a first terminal connected to first input of the short circuit comparison circuit, a second terminal connected to the first input of the over-current comparison circuit, and a third terminal connected to a ground terminal; and a delay circuit having an input connected to the output of the over-current comparison circuit and an output.
 23. The driver circuit of claim 22, wherein a sensed voltage proportional to a current through the power NFET is provided across the sense resistor.
 24. The driver circuit of claim 22, wherein the power NFET is a low-side NFET, the drain of the low-side NFET is connected to a switching terminal.
 25. The driver circuit of claim 22, wherein the power NFET is a high-side NFET, the drain of the high-side NFET is connected to a power supply terminal.
 26. The driver circuit of claim 22, wherein the source of the first NFET is connected to a switching terminal.
 27. The driver circuit of claim 22, wherein the source of the first NFET is connected to a ground terminal.
 28. The driver circuit of claim 22, wherein the voltage divider circuit is configured to apply a short circuit limit voltage to the first terminal of the short circuit comparison circuit.
 29. The driver circuit of claim 22, wherein the voltage divider circuit is configured to apply an over-current limit voltage to the first terminal of the over-current comparison circuit.
 30. The driver circuit of claim 22, wherein the short circuit comparison circuit is configured to provide a short circuit fault signal when the sensed voltage is greater than the short circuit limit voltage.
 31. The driver circuit of claim 22, wherein the over-current comparison circuit is configured to provide an over-current fault signal when the sensed voltage is greater than the over-current limit voltage.
 32. The driver circuit of claim 22, wherein the delay circuit is configured to provide a delayed over-current fault signal responsive to the over-current fault signal.
 33. A method of fault detection, comprising: providing a sensed voltage proportional to a current through a transistor; providing a short circuit limit voltage; providing an over-current limit voltage; comparing the sensed voltage to the short circuit limit voltage and the over-current limit voltage; providing a short circuit fault signal if the sensed voltage is greater than the short circuit limit voltage; providing an over-current fault signal if the sensed voltage is greater than the over-current limit voltage; and providing a delayed over-current fault signal by applying a delay to the over-current fault signal.
 34. The method of claim 33, further comprising turning off the transistor if the short circuit fault signal appears prior to the delayed over-current fault signal.
 35. The method of claim 33, further comprising turning off the transistor during the remainder of a current duty cycle and turning on the transistor in a next duty cycle if the delayed over-current fault signal appears prior to the short circuit fault signal. 